If you tried to play Pokemon Go this weekend, you probably already know Niantic, the company behind the wildly popular game, is struggling to keep it online. But that's not just a disappointment for the legions of players who have hit the streets looking for virtual monsters. It's also costing Niantic money: when Pokemon Go is down, no one can spend money in the game.

In the old days, all a game developer had to do was finish a game and then offer it for sale. Once customers plunked down their $50 for a Super Mario Brothers 3 game cartridge, they could play it as much as they wanted. Nintendo didn't have to worry about maintaining servers or offering security patches; it was up to you to blow on the cartridge if the game didn't work. But that also meant Nintendo didn't have a way to expand the game via online updates in ways that would allow it to make more money.

Today's mobile gaming world is completely different. Players expect games to be free, or so cheap that they might as well be. Instead, game companies rely on in-app purchases and upgrades to get players to cough up more cash.

That's the main way Niantic, a Google spin-off that has also received a significant investment from Nintendo, makes money from Pokémon Go. Players can buy a virtual currency called PokéCoins and exchange them for items within the game. And the sale of fake money could be a real bonanza for Niantic. Analytics firm AppAnnie estimates that Pokémon Go is already fetching $1 million a day and could eventually pull in $1 billion a year if it can work out its server issues while expanding into more countries and fleshing the game out a bit more.

But in-game purchases are just the beginning.